In accordance with recent increase in integration and capacity of large scale integration (LSI) circuits, the widths of circuit lines required for semiconductor devices has become increasingly smaller. Lithography technique is used to form a desired circuit pattern on a semiconductor device, and pattern transfer using an original drawing pattern referred to as a mask (reticle) is performed by using the lithography technique. To produce a high accuracy mask used in the pattern transfer, a charged particle beam drawing apparatus having excellent resolution is used.
The charged particle beam drawing apparatus draws a pattern on a substrate disposed on a stage by irradiating the substrate with a charged particle beam in a deflected manner by a charged particle optical system, while moving the stage that supports the substrate such as a mask or a blank in a chamber. To improve drawing accuracy, it is desired to minimize the distance between the charged particle optical system and the substrate, and the distance is preferably set to about a few millimeters (e.g., about 2 mm).
When the drawing is performed on the substrate with a charged particle beam by using such a charged particle beam drawing apparatus, there is a possibility that a conductive material, such as resist, formed on the substrate on a light shielding layer made of chromium (Cr) or the like may be charged. If the drawing continues in this state, the trajectory of the charged particle beam would be deflected by an electric field generated by the charged substrate. As a result of this, the drawing at desired positions is prohibited and the drawing accuracy is decreased.
To suppress the decrease of the drawing accuracy, a substrate cover disposed on the substrate to prevent charging of the substrate has been developed. A frame of the substrate cover is shaped like a frame to cover the surface circumference edge region of the substrate. On the frame, a plurality of conductive plates is provided, each having a pin that touches the surface of the substrate, and these conductive plates are connected to the ground. When such a substrate cover is disposed on the substrate, the pin of the conductive plate, which is grounded, touches the surface of the substrate to prevent charging of the substrate. The substrate cover also captures scattered electrons in the vicinity of the circumference of the substrate, such that the charging near the circumference edge of the substrate can also be prevented.
However, the distance between the charged particle optical system and the substrate becomes very narrow, as described above. If the thickness of the substrate cover remains thick as it is due to the conductive plate on the flame, the conductive plate of the substrate cover disposed on the substrate touches the lower end of the charged particle optical system during drawing. As a result of this, accurate drawing becomes impossible. Further, breakage of the charged particle optical system and the substrate cover is also concerned. It has been desired, therefore, to decrease the thickness of the substrate cover.